Diamond electrodes are distinguished by their high overvoltage for oxygen and hydrogen and are therefore particularly suitable for a wide range of oxidation processes in aqueous solution. Therefore, possible and particularly interesting applications would be in the field of drinking water treatment (disinfection) and water treatment by anodic oxidation. Further applications include electrochemical synthesis, in particular the production of oxidizing agents, and electrolysis of water and the electrochemical production of ozone and chlorine.
Currently, diamond electrodes are produced by direct generation of boron-doped diamond layers on substrate materials, in particular by CVD (chemical vapor deposition) processes. The known techniques differ from one another in particular with regard to the way in which the energy is introduced. In the case of what is known as hot-filament technology, tungsten wires are heated in a gas mixture of hydrogen, a carbon source and a boron source above a substrate. In this context, it is possible to use various substrates which in the CVD process differ in particular with regard to their different carbon solubility.
In practice, only copper and gold are inert. Titanium, zirconium, hafnium, niobium, tantalum, chromium, molybdenum, tungsten and silicon form carbide layers during the coating, which have to reach a certain material-specific thickness (a few nm in the case Si, a few μm in the case of Ti) before nucleation and therefore diamond deposition occurs. Even longer starting times during the CVD deposition occur with substrates which dissolve carbon, such as iron, cobalt, nickel, platinum and palladium. At the same time, during the coating reactions occur with the atomic hydrogen and carbon, forming carbide interlayers (SiO2, ZrO2). For these reasons, not all metals are equally suitable for coating.
When used as electrodes, it has emerged that various substrate materials which have a low oxygen overvoltage when connected as anode have an insufficiently long service life. If fine cracks are formed in the diamond layer, electrolysis oxygen is immediately formed, and this causes the diamond layer to become detached.
Boron-doped diamond layers produced using microwave plasma (MPCVD) can also be produced without a substrate, so that pure diamond electrodes of very high quality can be produced. To ensure a certain mechanical stability, electrodes of this type have to be produced in a relatively great layer thickness, which causes very high production costs and prevents economic use of this technology.
The invention is based on the object of providing and/or developing diamond electrodes and processes for producing diamond electrodes which make it possible to enable diamond electrodes to be produced at an economically acceptable outlay and accordingly at low cost. Therefore, the intention is for economic use of diamond electrodes to be possible for the first time in a wide range of oxidation processes in aqueous solution. In particular, correspondingly large-area electrodes should be producible, these electrodes being suitable in particular for drinking water treatment or wastewater treatment and/or electrochemical synthesis.